For years, Gmail has been reading users' e-mails to display relevant ads, but soon it'll go a step further by learning users' habits.

Under the existing system, Gmail ads are based solely on keywords in the message you're currently reading. In the coming months, Gmail will get smarter, borrowing methods from Priority Inbox to learn which ad topics are most important to each user.

A Google support page, first spotted by TechCrunch, says Gmail will pick up on signals, such as the number of e-mails you read on a given topic, to show fewer, but better ads. For instance, if you read a lot of e-mails about food and dining, you might see more ads for local restaurants. E-mails that you delete or ignore will factor less into the ads you see down the line.

Like the original Gmail advertising that rolled out in 2004, the new system could be kind of creepy at first. Gmail won't just be spitting back ads in the moment. It'll be building a profile of the things that are most important to you based on your personal correspondences.

But Google says the system is fully automated, and shares no personally identifiable information with advertisers. You'll also be able to turn off the ranking system under Gmail settings, although it will be enabled by default when it starts rolling out in a month or so. For these reasons, I'll bet the people who don't care will eventually drown out those who think these ads are a privacy outrage. And in any case, they'll be easier to ignore than display ads, which Google has also been testing recently.

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